Twelve experiments focus on the development, in human infants, of knowledge of physical objects and their behavior. The experiments investigate infants' sensitivity to five constraints on object motion: substance (objects only move through space not occupied by other objects), spatiotemporal continuity (objects only move on connected paths), gravity (objects are subject to a downward attraction), inertia (objects move on linear paths at constant velocity in the absence of forces), and force dependence (the direction and magnitude of changes in object motion are determined, respectively, by the direction and magnitude of the forces acting on the object.) Sensitivity to these constraints is investigated by means of preferential looking methods. In some experiments, infants are presented with an object that moves behind a screen, and then the screen is raised to reveal the object in a resting position that is either consistent or inconsistent with a given constraint on object motion. In other experiments, infants are presented with fully visible events in which objects are seen to move in ways that are either consistent or inconsistent with a given constraint. If infants appreciate a physical constraint on object motion, then they are expected to look longer at the inconsistent object position or motion. The long-term objectives of this proposal are (1) to elucidate core human conceptions of the physical world through studies of their origins in infancy, (2) to elucidate the organization of physical knowledge by investigating synchronies and asynchronies in the development of knowledge of different physical principles, and (3) to elucidate the mechanisms by which physical knowledge is acquired, through study of the acquisition process. An understanding of core physical knowledge, its organization and its development might shed light on the structure and acquisition of knowledge more generally, and it should aid efforts to facilitate knowledge acquisition in formal science instruction. In the future, such understanding might contribute to the detection and treatment of children with early developing deficits in cognitive functioning.